Creating GDM Themes HOWTO
Creating your own theme
It is always easier to start off of an existing theme, rather than doing it from scratch. In general, the first item to be changed is the background image. The background image must be at least as big as the screen resolution. In order to make the theme suitable to other users, it is a good idea to use at least 1600x1200 pixel images, as they look sharp even on high-resolution screens. A small image can also be used, but it will loose quality when expanded. The image must be placed in the theme directory and declared in the xml file as follows: